Among presently used types of amperometric gas sensors are those which include a gas permeable membrane that is located at one end of an internal chamber filled with an electrolyte solution such as a solution of potassium chloride in water. Immersed in the electrolyte solution is a cathode electrode which is located in the vicinity of the membrane to maximize its exposure to gas molecules diffusing therethrough. Surrounding the cathode electrode is an anode electrode which completes the circuit through the electrolyte solution. these electrodes are connected to an external measuring instrument which supplies the operating voltage between the electrodes and which provides a user readable indication of the current flowing therebetween.
In gas sensors of the above-described type which sense oxygen the cathode electrode is usually a metal disc or cap, composed of a noble metal such as gold, which is attached to the end of a cathode mounting element or post. The interior of this post carries the conductor that connects the cathode electrode to the external instrument. During assembly, the cathode electrode is sealed to the end of the cathode mounting post with an epoxy cement. Thereafter, the interior of the cathode mounting post is filled with a suitable potting compound, which may also be an epoxy type material.
While cathode assemblies of the above-described type often operate satisfactorily at first, their performance frequently deteriorates with time, giving rise to residual currents, i.e., currents which bear no meaningful relationship to the quantity of gas diffusing through the membrane. This deterioration has required that gas sensors which include such cathode assemblies be carefully monitored and frequently calibrated. This need for monitoring and calibration has, in turn, increased the cost of operating amperometric gas sensors and has rendered them unsuitable for use in unattended installations.
During the making of the present invention it was discovered that one reason for the above-described deterioration in the migration or diffusion of the electrolyte solution into and through defects in the bonds formed by the cement that is used to seal the electrode to its mounting post. This migration of the electrolyte solution also occurs along defects in the bond between the sealing cement and the internal potting compound. It was also discovered that once this electrolyte solution reaches the vicinity of the inner surface of the cathode electrode, serious current measurement errors begin to occur. If, for example, the conductor to which the cathode electrode is soldered contains the usual mixture of tin and lead, the presence of the electrolyte solution will initiate electrochemical reactions therebetween that affect the potential of the cathode electrode. In addition, if the deterioration of the sealing cement progresses far enough, metal ions which have gone into solution as the result of these electrochemical reactions will diffuse back into and contaminate the electrolyte solution.
Additional problems have been found to occur as the result of the contact between inwardly leaked electrolyte solution and the thermistor assembly that is usually soldered to the inner surface of the cathode electrode to provide information about the cathode operating temperature. Certain of these problems occur as a result of the above-described electrochemical reactions between metals in the thermistor leads, the thermistor mounting saddle and the solder used to fasten the latter to the cathode electrode. The presence of the electrolyte solution in the vicinity of the thermistor can also give rise to still further errors by supporting the flow of leakage currents between the leads of the thermistor and by establishing unintended leakage paths between the thermistor and the cathode electrode. The latter leakage currents are particularly objectionable since they give the appearance of being genuine signal currents.
The above-described problems are aggravated by the fact that, during normal operation, the electrochemical reactions occurring at the outer surface of the cathode electrode cause hydroxyl ions to be present in the vicinity of the seals between the cathode electrode and the cathode mounting post. These ions have been found to attack these seals by reacting chemically with the epoxy compounds included therein. As a result, originally insignificant defects can be converted into defects that cause serious measurement errors. Moreover, even if the cathode assembly is originally without sealing defects, the presence of hydroxyl ions can produce such defects and thereby give rise to the above-described errors.